Combination pickling and coating baths for ferrous metals and methods of using same



nited Star COMBINATION PICKLING AND COATING BATHS FOR FERRDUS METALS AND METHUDS OF USING SAME William (1. Johns and Wesley ,l'. Wojtowicz, Detroit, Micln, assignors to The H. A. Montgomery Company, Detroit, Mich, a corporation of Michigan No Drawing. Application January 23, 1956 Serial No. 56il,859

27 Claims. (Cl. 148-614) This invention relates to a method of preparing ferrous metal surfaces for drawing operations to be performed thereon. More particularly, the invention relates to a method of pickling ferrous metal surfaces to remove rust and scale and to form in situ on the pickled surfaces a chemically bonded coating serving both as a drawing aid and as a protective coating, all in the course of a single immersing operation.

Heretofore, it has been the practice first to perform the pickling operation, then to wash the surface to remove the pickling solution, then to perform the coating operation, and finally to wash the surface to remove the coating solution. Since each washing operation often required two separate irnmersions, it has been common practice to immerse ferrous metal articles as many as six or more times in six or more different tanks in the course of pickling and coating. This involves material processing times up to as high as 1 /2 to 2 hours, in some cases, and a large amount of processing equipment and plant floor space.

According to the practices of the prior art, in which separate pickling and coating operations were employed, it was diflicult to obtain uniform results without employing a relatively severe pickling operation to insure that every portion of the article to be subsequently coated had all rust, scale, and other foreign matter removed so as to present a perfectly clean surface of the base metal for coating. Such severe pickling operations are uneconomi cal, both in regard to pickling time and pickling acid consumption. On the other hand, inadequate pickling of any small surface areas was more serious than inadequate coating thickness, because any such areas containing scale residues would tend to abrade either the work surface or the working tool, or both. Also, such areas would have no securely bound coating whatsoever.

While efforts have heretofore been made to pickle and coat ferrous metals in one operation, they have been successful only where the metal surfaces were initially very clean, so that the degree of pickling required was not substantially greater than the normal attack on the surface of the metal inherent in the coating process itself (hereinafter explained). When any substantial pickling has been required to expose clean base metal to be coated, separate pickling and coating baths have generally been assumed to be necessary to the production of satisfactory coatings.

One of the principal objects of the present invention is to provide an improved bath composition and method for both pickling and coating ferrous metal articles in a single immersion operation, thereby eliminating as many as three of the prior art immersion steps.

Another of the principal objects of the invention is to provide controlled pickling in a combination pickling and coating bath so as to eliminate the danger of inadequate pickling and the objection of over-pickling on any part of the surface of the article to be coated, thereby insuring the application of a uniformly bonded coating over the entire surface of the article.

Still another object of the invention is to produce, in

2,846,341 Patented Aug. 5, 1958 64 a combination pickling and coating bath, more uniform and superior coatings on ferrous metal articles, which coatings provide a desirable interface to receive additional drawing lubricant and a more permanent and chemically resistant protective coating.

Still other objects of the invention are to reduce the floor space in a plant and the amount of equipment required for pickling and coating, to speed up those operations, and to generally reduce the cost of plant space, equipment, and equipment maintenance, all While providing more satisfactory coated metal articles.

In accordance with the present invention, pickling and coating are accomplished, in sequence, by a single immersion of ferrous metal articles in a heated aqueous solution comprising (a) from about 2% to about 15% of a strong mineral pickling acid, such as sulfuric or hydrochloric, and (b) oxalic acid from about 2% up to the limit of its solubility. To achieve more rapid coating, a more chemically inert coating, and greater coating thicknesses, the solution should also include from about 0.3% to about 5% or so of an oxidizing agent that is soluble in the solution, such as ferric oxalate, ferric chloride, ferric sulfate, and the like.

When such a bath is employed, with appropriate adjustment of proportions and immersion times according to the degree of pickling and coating thickness desired, the immersed ferrous metal articles first undergo a pickling action if they carry an oxide film of rust or scale. During the pickling action, a ferrous oxalate concentration builds up in the bath as a result of the attack of the bath on the rust and scale on the article. Any desired rate of pickling may be obtained over a wide range by varying the concentration and kind of mineral pickling acid employed. Moreover, until the base metal is exposed at any given point, generally no coating at that point occurs'.

When the pickling action has exposed the base metal over any given area, coating begins, and further pickling of that area is greatly reduced, due to the formation of the substantially acid-insoluble coating. In the absence of an oxidizing agent, some coating occurs, but the resultant coating appears to consist essentially of ferrous oxalate. Such a coating is of little value, either as a protective coating or as a drawing aid when the coated articles are subjected to drawing operations, with or without the use of an additional drawing lubricant. In the presence of one of the oxidizing agents mentioned above, such as ferric oxalate, the deposited coating is believed to be a ferrous-ferric oxalate complex of presently uncertain composition. This coating is largely insoluble in solutions which will dissolve simple ferrous oxalate coatings, such as dilute acetic acid, warm sodium acetate, and dilute acids in general (including nitric) and hence is a superior protective coating. Coating weights or thicknesses and the physical character of the coatings produced are variable according to time of immersion, temperature of the bath, and the concentration and proportions of the bath components, as hereinafter disclosed, and the resultant coatings form excellent aids in the retention of subsequently applied drawing lubricants and thereby contribute to the reduction of friction, prolongation of die life, and the production of better and more uniform products.

noted above, has little value as a rust inhibitor or coating for facilitating drawing operations, because of poor adherence to the base metal and the relatively thin coating deposit obtainable, as well as insufficient resistance to chemical attack. Also, the pickling action of a plain oxalic acid solution is so mild and erratic that it is entirely inadequate for most commercial pickling require ments.

As disclosed in U. 8. Patents No. 1,895,569 to Curtin and No. 2,273,234 to Tanner, ferric iron acts as an oxidizing agent when added to an oxalic acid solution in the form of a soluble salt. The addition thereof accelerates the coating action of the solution. As disclosed in both of said patents, the solution having this accelerated coating action must be acidic and must have a substantial concentration of both oxalate irons and ferric ions. As stated in the Tanner patent, the actual attack on the metal is evidently by the oxalic acid itself, and, therefore, there must be sufficient oxalic acid employed so that it is present as free acid after the formation of such oxalates as are in solution. In the presence of the ferric ion, the deposit takes a form which is relatively insoluble in solutions that will normally dissolve both ferrous oxalate and ferric oxalate, possibly due to the formation to some degree of a more insoluble ferrous-ferric oxalate complex. The ferric ion in solution apparently acts as an oxidizer to supplement the action of the oxalic acid in converting metallic iron from the metal surface to ferrous iron in. solution. In doing so, it accelerates the coating action and is itself reduced to form additional ferrous iron in solution. Thus, the coating is formed partly at the expense of the ferric and oxalate irons in the solution and only partly by attack on the metal surface. Though the addition of a ferric salt to an oxalic acid bath accelerates the coating, it does not cause the bath to have any appreciably greater pickling action.

The Curtin patent also points out the tendency of ferric oxalate to hydrolyze in water and produce ferric hydroxide, which is poorly ionized and tends to contaminate the coating. Both to retard such hydrolysis and to facilitate the above described oxidizing function of the ferric ion in solution, the Curtin patent suggests the desirability of maintaining a certain degree of acidity in the treating solution. However, Curtin also explains that the quantity of free acid in the bath should not be sufiicient to give free evolution of hydrogen by reaction with the iron of the article being treated, i. e., it should not exhibit substantial pickling properties. To this end, the Curtin patent suggests the use of sulfuric acid in a concentration of only 0.5%, or an oxalic acid concentration of about 1% to a maximum of about 5%. In another U. S. patent of the same date, No. 1,895,568, directed to a similar bath and process, Curtin suggests oxalic acid concentrations up to about along with about 0.5% to 1% sulfuric acid. Such low sulfuric acid concentrations alter the chemical character of the coatings somewhat, but with little improvement in its chemical inertness. They accelerate the coating action, and, to about the same degree, they accelerate the concurrent pickling action which appears to be an inherent part of the coating process itself, as distinguished from the successive pickling and coating action of baths made in accordance with the present invention.

Though these prior art patents, and others on related processes, are somewhat obscure as to the exact role played by the acid concentration of the coating solution, whether consisting solely of oxalic acid, solely of sulfuric acid, or containing both, the prior teachings of the art have been uniformly to the effect that the acid concentration in a coating bath should be sufliciently low to avoid any substantial pickling action. Obviously, some attack on the iron surface, which puts iron into solution (with evolution of hydrogen), is essential to the described coating action. However, even with the amount of sul- 4 furic acid (0.5% to 1.0%) suggested in Curtin Patent 1,895,568, the pickling action is sufficient to dissolve only a little rust. Such low concentrations of sulfuric acid are of relatively little value for this purpose. Thus, in all of these cases, iron and steel articles requiring any substantial pickling before coating have required preliminary pickling with a suitable pickling acid before immersion in a coating bath.

One reason given by the prior art for avoiding any substantial concentration of an effective pickling acid in a coating bath is that it causes a high rate of evolution of hydrogen during the coating process, as a result of attack by the pickling acid on the iron surface, which tears or disrupts the coating. Also, it is well known that oxalic acid tends to decompose on heating and that such decomposition is promoted by sulfuric acid, which is the most commonly used pickling acid for ferrous metals.

See page 309, of Organic Chemistry by Paul Karrer,

1950. Probably for these additional reasons, no effort has been made to combine a sufiicient concentration of sulfuric acid for effective pickling with an oxalic acid coating bath, and the concentration of sulfuric acid used in such baths has apparently been held to a maximum of about 1% or less. As noted above, a sulfuric acid concentration as low as 1%, when incorporated in any of the above described prior art coating baths, is too low for any effective pickling in the sense of removing rust and scale prior to coating.

In U. S. Patent No. 2,712,511 to Orozco et al., granted July 5, 1955, a coating bath for ferrous metals is described which contains oxalic acid up to a maximum of about 11% by weight as at least the principal film forming ingredient, phosphoric acid up to a maximum of about 4 /2% by weight as a so-called etching and probably also film forming ingredient, and hypophosphorous acid up to a maximum of about 0.4% by weight as an accelerator. It is speculated that the resultant coating may be an iron phospho-oxalate film, which is said to be microporous and chemically bonded to the surface of the stock. Nothing is said about pickling, as the term is normally used to denote the cleaning of rust and scale from the metal stock, and, with the concentrations of phosphoric and hypophosphorous acid suggested, no substantial pickling action could occur. The etching action referred to in this patent is apparently nothing more than the necessary attack on the base metal to provide the iron component of the coating. In fact, it is questionable whether all or a major part of the coating action is effected by the oxalic acid content, and similarly, whether all or a major part of the etching action is performed by the phosphoric acid present. Actually, the greater degree of ionization of oxalic acid as compared to phosphoric acid in the concentrations expressed would lead to the belief that the reverse is true, i. e., the oxalic acid reacts first to etch and form ferrous iron which combines either separately with each acid to form a mixture of ferrous oxalate and ferrous phosphate as the coating, or simultaneously with each acid to form a complex phosphooxalate film.

By employing a bath comprising oxalic acid up to the limit of its solubility and an oxidizing salt providing or creating ferric ions in solution, together with sulfuric acid in a concentration sufiicient to have a pronounced pickling eifect (preferably 5% or more), the present invention achieves several new and improved results:

First, depending upon the particular sulfuric acid concentration, any desired severity of pickling canbe obtained which is obtainable with a plain sulfuric acid prepickling operation. This pickling is accompanied by evolution of hydrogen at rates comparable to those occurring in plain sulfuric acid pre-pickling operations, a phenomenon formerly considered to be seriously detrimental in a coating bath. A

asaaaai Second, the pickling action is self-regulating in that it automatically diminishes upon exposure of clean base metal in any area, whereupon coating over that area automatically commences. Thus, the economic objection of overpickling and the practical danger of underpickling are both avoided merely by employing an initial sulfuric acid concentration appropriate for the severity of the pickling required in view of the amount of rust and scale to be removed.

Third, instead of interfering with the subsequent coating action of the oxalic acid and any accelerator employed, the high sulfuric acid concentration actually improves the character of the coating that is formed, with an actual acceleration of the coating rate, or, at least, no significant retardation (making due allowance for the time interval during which pickling is occurring prior to the beginning of the actual coating action).

Though comparison of the present invention with the various prior art processes mentioned above necessarily varies from case to case regarding the character of the coatings and the coating thicknesses produced, in all cases the process of the present invention either produces heavier coatings, or more chemically inert coatings, or both, with equal or superior physical properties as an interface for subsequently applied lubricant films.

Sulfuric acid has been principally referred to in the foregoing discussion as the strong mineral acid component of the bath compositions of the invention, and it is preferred for all ferrous metals except those requiring unusually strong pickling solutions. However, as previously noted and more fully explained hereinafter, hydrochloric acid may also be used.

From the foregoing discussion, it will be recognized that the present invention is characterized by substantial departures and differences in bath composition and in the results achieved thereby compared to prior art practices in the production of oxalate coatings.

With the foregoing general description of the objects, advantages, and characteristic features of the invention in mind, a complete understanding of the invention and its many permissible variations will be more readily obtained from the following examples of the action of various bath compositions and examples of preferred compositions for typical pickling and coating jobs. Where not otherwise indicated, all concentrations are given in terms of percent by weight.

EXAMPLE 1 To illustrate the relative activity of various prior art coating baths and two additional coating baths prepared in accordance with the invention, five separate baths were prepared. Baths A, B, and C represent three prior art compositions, and baths D and E represent those of the present invention. All five baths 'were tested for their action at about 170 F. on clean iron plates, to eliminate uncontrollable variations in conditions due to the nonuniformity of rust and scale formations. The bath compositions of the five baths, and the metal pickle losses and oxalate coating weights resulting from their pickling and coating actions on the test plates during equal periods of immersion are shown in Table I. Pickle losses and coating weights are both given in terms of milligrams per square foot of plate surface acted upon.

Table l B th Bath Composition, Percent by Weight Pickle Coating Loss Weight HzSO4 112C204 Fez(C2Ol)a eASOm From the data in Table I, it will be apparent that baths D and E conforming to the 'present invention provided a much stronger pickling action and an equal or more rapid coating action than any of the prior art baths A, B, and C. A comparison of the results obtained with baths A and D, which differed only in the sulfuric acid concentration also shows the above described effects of simply raising the acid concentration to a much greater value than has been used by the prior art.

In addition to the effects of the invention on pickling action and rate of coating, however, it is significant that there is also a change in the chemical character of the coating. This is evidenced by the solubilities of the coatings produced by all five of the baths in the above tests. The solubilities of those coatings in both acetic acid and sodium acetate were measured, and the coatings from baths D and E were found to be less soluble in both reagents, and particularly in the former, than the coatings of baths A, B, and C. The differences in coating compositions indicated by the differences in solubilities apparently account, at least in part, for the superior results of the invent-ion as regards lubricant retention and reduction of friction, as well as the provision of durable protective coatings during storage and handling of coated metal sheets or articles. The coatings produced in accordance with the invention also form excellent bases for other protective coatings, such as paints and lacquers.

From the foregoing and additional tests too numerous to describe herein, the general effects of various modifications of bath compositions and concentrations of ingredients can be given.

The sulfuric acid component of the bath is the effective pickling agent. Though this acid is not highly active for this purpose at concentrations below about 5% by weight, lower concentrations down to about 2% by weight may be used where only a very mild pickle is required. The maximum sulfuric acid concentration will generally be around 15% where severe pickling is required. The normal sulfuric acid concentration for treating iron and most low alloy steels is in the range of about 5% to 12%. The rate and severity of pickling increase with the concentration of sulfuric acid, but it should be noted that somewhat higher sulfuric acid concentrations should be employed than in a plain pickling bath because the oxalic acid has a mild inhibiting effect on the pickling activity of the sulfuric acid.

When treating stainless steels, which are highly resistant to pickling, hydrochloric acid may be substituted in whole or in part, preferably only in part, for the sulfuric acid. In general, the use of hydrochloric acid is undesirable where it is not essential to obtain satisfactory pickling, since the rate of coating is retarded somewhat and the quality and adherence of the coating are detrimentally affected. However, as indicated, the use of at least some hydrochloric acid is necessary in some cases to obtain adequate pickling, and its use is intended to be within the scope of the invention. In commercial operations, where hydrochloric acid in the bath is used in combination with sulfuric acid, the required total hydrogen ion concentration is preferably supplied as sulfuric acid and enough soluble chloride salt, such as sodium chloride, is added to provide the chloride ion. Anywhere from 2 or 3 to as much as 10% sodium chloride by weight, may suitably becombined with 10% by weight of sulfuric acid in such a case, depending upon the severity of the pickling requirements.

The oxalic acid is a component of the bath which is essential to the formation of a coating on the metal. However, it also exerts a pickling action and, in fact, unless it is reactive with the base metal, it cannot cause a coating to be deposited.

For some uncertain reason, no coating occurs on a given area until the pickling action of the mineral and oxalic acids together has exposed a clean base metal surface, whereupon pickling substantially ceases and the coating action begins. Thus, the bath composition of the invention is to a large degree self-regulating in limiting the pickling action so that hydrogen evolution does not destroy the coating. The oxalic acid appears to act first on the base metal (when exposed) to build up a concentration of ferrous ions in the solution. Until the concentration of ferrous ions in solution reaches saturation, no coating can occur.

The concentration of oxalic acid may be as high as its limit of solubility will permit. However, with high oxalic acid concentrations, higher sulfuric acid concentrations are required to obtain a given degree of pickling. Because the loss of active ingredients due to physical entrainment of the bath on the surfaces of treated articles naturally increases with the concentration of the ingredients, considerations of economy dictate the use of concentrations no higher than necessary to produce the desired results. Close to maximum coating weights are obtainable with oxalic acid concentrations of about and about an equal concentration of sulfuric acid. Raising only the oxalic acid concentration to as high as 10% while leaving the sulfuric acid concentration unchanged adds but little to the coating weight While substantially reducing the pickling action. Raising only the sulfuric acid concentration to as high as 10% while leaving the oxalic acid concentration unchanged greatly increases the pickling action and drastically reduces the coating action. On the other hand, nearly the same increase in pickling action may be obtained with a slight increase in coating action by raising the concentrations of oxalic and sulfuric acids by the same amount. Thus, for most purposes, substantially equal concentrations of the two acids are preferred, the amount being determined principally by the degree of pickling required.

While the simple combination of the sulfuric and oxalic acids (no oxidizing agent) will both pickle and coat to a limited extent, the coating Weights are very low compared to the results obtained when an oxidizing agent is also present, the pickling and coating actions gradually lessen with use of the bath, and the coating produced, being composed entirely of ferrous oxalate, lacks satisfactory adhesion and resistance to atmospheric oxidation. Thus, the presence of an oxidizing agent is highly important to practical commercial utility of the invention.

As noted above, the various soluble ferric iron salts, such as ferric oxalate, ferric sulfate, ferric chloride, and the like are suitable oxidizing agents. To prevent superfluous contamination of the coating bath solution or the color of the coating produced thereby, certain highly oxidizing anions and cations, such as manganic, chlorate, perchlorate, dichlormate, permanganate, and the like may be eliminated from consideration. Also, many other highly oxidizing anions and cations are insoluble in the sulfuric-oxalic acid solutions employed. Thus, the particular soluble ferric salts mentioned are the preferred oxidizing agents. While they are preferably added as such when preparing a fresh bath in accordance with the invention, it should be noted that the corresponding ferrous salts may be added and the ferrous ions oxidized to ferric ions by the inclusion of hydrogen peroxide or other similar non-contaminating oxidizing agent. Thus, the requirements for a satisfactory bath will be understood to be based on the composition of the resultant solution as distinguished from the manner in which such combinations of ingredients are brought together in solution.

As between the particular ferric salts mentioned above, the choice is determined by economic as well as practical considerations. Ferric chloride not only produces ferric ions in solution, but it also provides hydrochloric acid by chemical equilibrium, and this acid is generally not desired except where required in treating stainless steels and the like, as explained above. However, where this acid is required, ferric chloride would be entirely satisfactory, except for the fact that it is obtainable only in a caked form and not as a flake, crystal, or powder, and presents a handling problem, though otherwise operative. As between ferric oxalate and ferric sulfate, they may be used alone or in admixture. The use of sulfate in place of oxalate increases the sulfate ion concentration and, therefore, has generally the same effect as increasing the ratio of sulfuric to oxalic acids.

The effect of the oxidizing agent has been explained above, as fully as it is understood. Why the coating that is formed in the presence of an added soluble ferric salt is superior in its physical and chemical properties to a coating formed without the presence of the added ferric salt is still obscure, although the formation of a ferroferric-oxalate complex coating may account for this superiority. However, the action of the ferric salt in accelerating the coating action is attributable to its power to oxidize metallic iron of the treated metal and thereby accelerate the formation of ferrous oxalate in solution at the expense of the treated metal.

The addition of as little as 0.5% of the ferric salt is effective for this purpose. However, since it is slowly exhausted as the solution is used, it is desirably added to the solution up to about 2% or so. When replenishment of the ferric salt is required after a period of use, the concentration of ferrious ions in solution is high, and the simple addition of hydrogen peroxide serves the same function as the addition of more ferric salt. For most purposes, an initial concentration of 0.75 to 1.25% of the ferric salt is sufficient to provide optimum activity and coating quality and to minimize the frequency of re plenishment of this ingredient. Higher concentrations than 2% are of limited value.

To illustrate the above described effects of varying the concentrations of the three essential bath ingredients, reference is made to the following additional comparative example.

EXAMPLE 2 Eight different baths were prepared in accordance with the invention using different concentrations of sulphuric acid, oxalic acid, and ferric oxalate. Using clean iron test plates in the same manner as in Example 1, pickle losses and coating weights produced by each bath at about F. were determined after immersion of the plates for live minutes. The following Table II gives the concentrations of the above three ingredients in percent by weight and the calculated pickle losses and coating weights in milligrams per square foot of plate surface.

Table 11 Bath N0. Sulf. Oxalic Ferric Pickle Coating Acid Acid Oxalate Loss Weight 5 5 l 732 535 10 5 1 1, 650 54 5 1O 1 555 595 10 10 1 l, 483 681 5 5 2 733 450 10 5 2 2, 078 713 5 l0 2 644 563 10 10 2 1,092 469 A reducing salt, such as an alkali metal oxalate, appears to have some damping effect upon the pickling action of the oxalic acid, enabling it to act more as a coating agent, so as to deposit coatings of somewhat greater thickness in a given length of time. It also has an effect on the quality of the coating by giving the coating somewhat greater anti-friction properties in metal drawing operations. Sodium oxalate is the most satisfactory reducing salt from the standpoint of availability and cost, though potassium oxalate, for example, is equally effective.

Since good results are achieved without the reducing salt, it may be a desirable ingredient in some cases, but is not an essential ingredient. Where desired, it may be used in any amount up to a concentration of about 2% by 9 weight. In general, its use is preferred in amounts from about 0.5% to 1% by weight.

Presence of the manganous on in the pickling and coating solution appears to contribute to the corrosion resistance of the deposited coating and also to its lubricity in a metal drawing operation. In its latter effect, it performs somewhat the same function as the alkali metal oxalate. Thus, when a soluble manganous salt is included, the presence of an alkali metaloxalate becomes of little value. Whereas the alkali metal oxalate appears not to enter into the deposited coating, the manganese does, presumably in the form of an iron-manganese complex, and this is facilitated if the manganese salt employed is also an oxalate.

The temperature at which the pickling and coating baths of the invention are maintained in use may vary from as low as 130 F. to as high as 200 F. while obtainingthe desired results to a useful degree. However, a temperature in the range of about 160 F. to 180 F. is preferred.

Considering the three principal components of bath compositions embodying the persent invention, namely, the sulfuric and/ or hydrochloric acid, the oxalic acid, and the oxidizing metal salt, the relative proportions in which they are employed Within the individual ranges mentioned above are also subject to considerable variation and generalized statements can only be made with certain qualifications. However, the following results of variations in relative proportions will usually hold true:

With low oxalic acid concentrations. e. g., and moderate oxidizing salt concentrations, e. g., 1%, increasing the concentration of sulfuric acid from say, 5% up to decreases the coating thickness though it accentuates the pickling action. With the same low oxalic acid concentration and a higher concentration of oxidizing salt, however, increasing the sulfuric acid concentration from, say, 5% up to 10%, increases both the coating thickness and pickling action.

On the other hand, with substantially equal concentrations of sulfuric acid and oxalic acid, the optimum concentration of oxidizing salt is about 1%, and higher concentrations tend to reduce both the coating and pickling action.

If the sulfuric acid concentration is relatively low, e. g., 5%, increasing the oxalic acid concentration from, say, 5% to 10%, only moderately increases the coating thickness and, to a somewhat greater degree, suppresses the pickling action.

In general, where the objective is to produce a protective film of substantial thickness as a base for receiving a drawing lubricant and/or as a protective coating, the concentration of-sulfuric acid should be sufiicient to provide necessary pickling in a satisfactory time period, and the oxalic acid concentration is about the same as the sulfuric acid concentration. Higher concentrations of oxalic, acid than sulfuric acid are of no particular benefit, except where very. little pickling is necessary. Where necessary, as in treating stainless steels, hydrochloric acid may be largely substituted for sulfuric acid, and the total acid concentration may be raised to as high as 25% or more, though the coating thickness is progressively reduced with acid concentrations above about 10% to I Illustrative bath compositions for most commercial needs would be as follows:

EXAMPLE 3 For uses Where only slight pickling and a light coating are required, 2% sulfuric acid, 2% oxalic acid, and /2% ferric oxalate or sulfate, alone or combined, are preferred to higher concentrations with shorter immersion times principally from economic considerations.

EXAMPLE 4 For uses where moderate to strong pickling action and moderate to heavy coatings are required, 5% sulfuric to acid, 5% oxalic acid, and 1% ferric oxalate or ferric sulfate, alone or combined, are preferred with immersion times adjusted to suit the needs.

EXAMPLE 5 For uses Where extreme pickling action is required, as in the case Of heavily scaled steel of the non-stainless variety, 10% sulfuric acid, 10% oxalic acid, and 1% ferric oxalate or ferric sulfate, alone or combined, are preferred, with immersion times adjusted to suit the needs.

EXAMPLE 6 For treating stainless steels such as the 400 series,

the composition of Example 5 may be used. However,

still greater pickling action may be required and is obtainable by increasing the ferric salt concentration to 2%.

EXAMPLE 7 For still greater pickling action on the more resistant stainless steels, such as the 300 series, the use of hydrochloric acid may be required. In such cases, 10% sulfuric acid, 5% sodium chloride to produce hydrochloric acid in the solution, 10% oxalic acid, and 215% ferric oxalate, or ferric sulfate, or ferric chloride, or any mixture thereof may be used. The severity of the pickling action increases as the concentration of ferric salt is increased from 2% to 15%, and such increases in ferric salt concentration are more effective for that purpose than increasing the sulfuric acid and/or sodium chloride concentrations.

From the foregoing discussion, it will be apparent that the composition of pickling and coating. baths in accordance with the invention may be varied considerably to meet specific requirements imposed by different kinds. of ferrous metals to be treated, different kinds of forming operations, different corrosion resistance problems, etc.

Having described our invention, we claim:

1. In the treatment of ferrous metal to pickle and coat the same, the step of immersing the metal in a heated pickling bath comprising an aqueous solution of from about 5% to about 15% by weight of pickling mineral acid selected from the class consisting of sulfuric and hydrochloric acids and mixtures thereof, from about 2% by Weight up to the limit of its solubility of oxalic acid, and at least 0.5% by weight of soluble ferric iron salt, and continuing the immersion until the desired degree of pickling and coating have occurred.

2. In the treatment of ferrous metal to pickle and coat the same, the step of immersing the metal in a heated pickling bath comprising an aqueous solution of from about 5% to about 15% by weight of pickling mineral acid selected from the class consisting of sulfuric and hydrochloric acids and mixtures thereof, from about 2% by weight up to the limit of its solubility of oxalic acid, and from 0.5% to 15 by weight of soluble ferric iron salt, and continuing the immersion until the desired degree of pickling and coating have occurred.

3. In the treatment of ferrous metal to pickle and coat the same, the step of immersing the metal in a heated pickling bath comprising an aqueous solution of from about 5% to about 15% by weight of pickling mineral acid selected from the class consisting of sulfuric and hydrochloric acids and mixtures thereof, from about 2% by weight up to the limit of. its solubility of oxalic acid, and from 0.5% to 2% by weight of ferric iron salt selected from the class consisting of ferric oxalate, ferric sulfate, and ferric chloride, and continuing the immersion until the desired degree of pickling and coating have occurred.

4. In the treatment of ferrous metal to pickle and coat the same, the step of immersing the metal in a heated pickling bath comprising an aqueous solution of from about 5% to about 15% by Weight of pickling mineral acid selected from the class consisting of sulfuric and 1 1 hydrochloric acids and mixtures thereof, from about 2% by weight up to the limit of its solubility of oxalic acid, and from about 0.5% to 2% by Weight of a mixture of ferric oxalate and ferric sulfate, and continuing the immersion until the desired degree of pickling and coating have occurred.

5. In the treatment of ferrous metal to pickle and coat the same, the step of immersing the metal in a heated pickling bath comprising an aqueous solution of from about 5%, to about 15% by weight of pickling mineral acid selected from the class consisting of sulfuric and hydrochloric acids and mixtures thereof, from about 2% by weight up to the limit of its solubility of oxalic acid,

and from about 0.5% to 2% by Weight of ferric oxalate,

and continuing the immersion until the desired degree of pickling and coating have occurred.

6. In the treatment of ferrous metal to pickle and coat the same, the step of immersing the metal in a heated pickling bath comprising an aqueous solution of from about 5% to about by weight of pickling mineral acid selected from the class consisting of sulfuric and hydrochloric acids and mixtures thereof, from about 2% by weight up to the limit of its solubility of oxalic acid, and from about 0.5% to 2% by weight of ferric sulfate, and continuing the immersion until the desired degree of pickling and coating have occurred.

7. In the treatment of ferrous metal to pickle and coat the same, the step of immersing the metal in a heated pickling bath comprising an aqueous solution of A from about 5% to about 10% each by weight, of sulfuric and oxalic acids and from about 0.5 to 2% by Weight of soluble ferric iron salt, and continuing the immersion until the desired degree of pickling and coating have occurred.

8. A combination pickling and coating bath for ferrous metals, consisting essentially of an aqueous solution of from about 5% to about 15% of pickling mineral acid selected from the class consisting of sulfuric acid,

hydrochloric acid, and mixtures thereof, from about 2% by weight up to the limit of its solubility of oxalic acid, and at least 0.5% by weight of soluble ferric iron salt.

9. A combination pickling and coating bath for ferrous metals consisting essentially of an aqueous solution of from about 5% to about 15 of pickling mineral acid selected from the class consisting of sulfuric acid, hydrochloric acid, and mixtures thereof, from about 2% by weight up to the limit of its solubility of oxalic acid, and from about 0.5% to 2% by weight of ferric iron salt selected from the class consisting of ferric oxalate, ferric sulfate, and ferric chloride.

10. A combination pickling and coating bath for ferrous metals consisting essentially of an aqueous solution of from about 5% to about 15% of pickling mineral acid selected from the class consisting of sulfuric acid, hydrochloric acid, and mixtures thereof, from about 2% by weight up to the limit of its solubility of oxalic acid, and from about 0.5% to 2% by weight of a mixture of ferric oxalate and ferric sulfate.

11. A combination pickling and coating bath for ferrous metals consisting essentially of an aqueous solution of from about 5% to about 15% of pickling mineral acid selected from the class consisting of sulfuric acid, hydrochloric acid, and mixtures thereof, from about 2% by weight up to the limit of its solubility of oxalic acid, and from about 0.5% to 2% by weight of ferric oxalate.

12. A combination pickling and coating bath for ferrous metals consisting essentially of an aqueous solution of from about 5% to about 15% of pickling mineral acid selected from the class consistingof sulfuric acid,

hydrochloric acid, and mixtures thereof, from about 2% by weight up to the limit of its solubility of oxalic acid, and from about 0.5 to 2% by weight of ferric sulfate.

13. A combination coating and pickling bath for ferrous metals, consisting essentially of an aqueous solution of from about 5% to about 15% by weight of sulall) . l2 furic acid, from about 2% by weight up to the limit of its solubility of oxalic acid, and at least 0.5% by weight of soluble ferric iron salt.

14. A combination coating and pickling bath for ferrous-metals, consisting essentially of an aqueous solution of from about 5% to about 10% by weight of sulfuric acid, from about 2% by weight up to the limit of its solubility of oxalic acid, and at least 0.5 by weight of soluble ferric iron salt. 4

I 15. In the treatment of ferrous metal to pickle and coat the same, the step of immersing the metalin a heated pickling and coating bath comprising an aqueous solution of substantially equal parts of sulfuric acid and oxalic acid, each in an amount of from about 5% to about 10% by weight, and from about 0.5% to 2% by weight of a ferric iron salt selected from the class consisting of ferric oxalate, ferric sulfate, and ferric chloride, and continuing the immersion until the desired degree of pickling and coating have occurred.

16,111 the treatment of ferrous metal to pickle and coat the same, the step of immersing the metal in a heated pickling and coating bath comprising an aqueous solution of substantially equal parts of sulfuric acid and oxalic acid, each in an amount of from about 5% to about 10% by weight, and from about 0.5 to 2% by weight of a mixture of ferric oxalate and ferric sulfate, and continuing the immersion until the desired degree of pickling and coating have occurred.

17. In the treatment of ferrous metal to pickle and coat the same, the step of immersing the metal in a heated pickling and coating bath comprising an aqueous solution of substantially equal parts of sulfuric acid and oxalic acid, each in an amount of from about 5% to about 10% by weight, and from about 0.5% to 2% by Weight of ferric oxalate, and continuing the immersion until the desired degree of pickling and coating have occurred.

18. In the treatment of ferrous metal to pickle and coat the same, the step of immersing the metal in a heated pickling and coating bath comprising an aqueous solution of substantially equal parts of sulfuric acid and oxalic acid, each in an amount of from about 5% to about 10% by weight, and from about 0.5% to 2% by weight of ferric sulfate, and continuing the immersion until the desired degree of pickling and coating have occurred.

19. In the treatment of ferrous metal to pickle and coat the same, the step of immersing the metal in a heated pickling and coating bath comprising an aqueous solution of sulfuric and hydrochloric acids ina total concentration of at least 10%, oxalic acid in an amount of at least 2% by weight, and at least 2% by weight of soluble ferric iron salt, and continuing the immersion until the desired degree of pickling and coating have occurred.

20. In the treatment of ferrous metal to pickle and coat the same, the step of immersing the metal in a heated pickling and coating bath comprising an aqueous solution of sulfuric and hydrochloric acids in atotal concentration of at least 10%, oxalic acid in an amount of at least 2% by weight, and at least 2% by weight of ferric iron salt selected from the class consisting of ferric oxalate, ferric sulfate, and ferric chloride, and continuing the immersion until the desired degree of pickling and coating have occurred.

21. A combination coating and pickling bath for ferrous metals, consisting essentially of an aqueous solution of substantially equal amounts of sulfuric acid and oxalic acid, each in an amount of from about 5% to about 10% by weight, and from about 0.5% to about 2% by weight of soluble ferric iron salt.

22. A combination coating and pickling bath for ferrous metals, consisting essentially of an aqueous solution of substantially equal amounts of sulfuric acid and oxalic acid, each in an amount of from about 5% to about 13 10% by Weight, and from about 0.5 to about 2% by weight of soluble ferric iron salt selected from the class consisting of ferric oxalate, ferric sulfate, and ferric chloride.

23. A combination coating and pickling bath for ferrous metals, consisting essentially of an aqueous solution of substantially equal amounts of sulfuric acid and oxalic acid, each in an amount of from about 5% to about 10% by weight, and from about 0.5% to about 2% by weight of a mixture of ferric oxalate and ferric sulfate.

24. A combination coating and pickling bath for ferrous metals, consisting essentially of an aqueous solution of substantially equal amounts of sulfuric acid and oxalic acid, each in an amount of from about 5% to about 10% by weight, and from about 0.5% to about 2% by weight of ferric oxalate.

25. A combination coating and pickling bath for ferrous metals, consisting essentially of an aqueous solution of substantially equal amounts of sulfuric acid and oxalic acid, each in an amount of from about 5% to about 10% by weight, and from about 0.5 to about 2% by weight of ferric sulfate.

26. A combination coating and pickling bath for ferrous metals consisting essentially of an aqueous solution of at least 10% by weight of a mixture of sulfuric and hydrochloric acids, from 2% by Weight up to the limit of its solubility of oxalic acid, and from 2% to 15% by weight of soluble ferric iron salt.

27. A combination coating and pickling bath for ferrous metals consisting essentially of an aqueous solution of at least 10% by weight of a mixture of sulfuric and hydrochloric acids, at least 10% by weight of oxalic acid, and from 2% to 15% by weight of a ferric iron salt selected from the class consisting of ferric oxalate, ferric sulfate, and ferric chloride.

References Cited in the file of this patent UNITED STATES PATENTS 1,146,071 Hoffman July 13, 1915 1,895,568 Curtin Jan. 31, 1933 2,273,234 Tanner Feb. 17, 1942 

1. IN THE TREATMENT OF FERROUS METAL OF PICKEL AND COAT THE SAME, THE STEP OF IMMERSING THE METAL IN A HEATED PICKLING BATH COMPRISING AN AQUEOUS SOLUTION OF FORM ABOUT 5% TO ABOUT 15% BY WEIGHT OF PICKLING MINERAL ACID SELECTED FROM THE CLASS CONSISTING OF SULFURIC AND HYDROCHLORIC ACIDS AND MIXTURES THEREOF, FROM ABOUT 2% BY WEIGHT UP TO THE LIMIT OF ITS SOLUBILITY OF OXALIC ACID, AND AT LEAST 0.5% BY WEIGHT OF SOLUBLE FERRIC IRON SALT, AND CONTINUING THE IMMERSION UNTIL THE DESIRED DEGREE OF PICKING AND COATING HAVE OCCURRED. 